That case looks nice, even has an TG panel option! although around £150 is a lot

Also im assuming that the airflow will be sacrificed so not as good cooling = noisier fans?

Also I'v just realised that the 240 AIO would be mounted in front of the mobo so there would be no point to have hte window

No, that’s correct... Although, to be completely honest here, you seem to have too high expectations. A small build WILL be compromisses, that’s just how it really works. Unfortunately that’s just how it is, you can’t have it all.

ASRock is the best built ITX board. ASUS is probably the most optimized in BIOS and features. And Gigabyte well we all know their BIOS isn't that good.

For connectivity ASRock has Thunderbolt built in. It is 20 Gb/s instead of 40 Gb/s TB but for USB connectivity you can just buy HUB and connect anything that you need.

I would suggest that you turn back case fan to push air in rather than pull it out. In that case you will have front AIO in, back fan in, blowing air directly on the VRM and 2 fans top for exhaust. If you set it up that way any board will do fine.

Also you can change thermal pads on VRM heatsinks. Der8auer did testing with X299 and just buy replacing stock pads with ThermalGrizzly ones he got better temps. Make sure you clean heatsink and VRMs with alcohol and ALWAYS use same thickness pads to replace the stock ones.